1. Field of the Invention
The invention relates to hydrolyzing lignocellulosic materials by subjecting milled lignocellulosic material in a reactor to a catalyst comprised of a dilute solution of a metal salt and a strong acid to lower the activation energy (i.e. temperature) of cellulose hydrolysis compared to that normally required for dilute acid hydrolysis. The lower temperature obtained occasions a reduction in the cost of steam and equipment and enables the hydrolysis of both hemicellulose and cellulose to proceed in a single stage with higher glucose yield from cellulose and less degradation of the hemicellulose sugar. The lower temperature also occasions a higher glucose yield in the second-stage of the two-stage dilute acid hydrolysis process.
Lignocellulose is ubiquitous in all wood species and all agricultural and forestry waste. In addition, municipal waste, which typically contains about half waste paper and yard waste, is a source of lignocellulosic materials. Currently, municipal waste is buried or burned at considerable expense to the disposer or the government organization providing solid waste services.
Lignocellulosic biomass is a complex structure of cellulose fibers wrapped in a lignin and hemicellulose sheath. The ratio of the three components varies depending on the type of biomass. Typical ratios are as follows:
Different woods also have different compositions. Softwoods (gymnosperms) generally have more glucommanan and less glucuronoxylan than hardwoods and grasses (angiosperms).
Cellulose is a polymer of D-glucose with xcex2[1xe2x86x924] linkages between each of the about 500 to 10,000 glucose units. Hemicellulose is a polymer of sugars, primarily D-xylose with other pentoses and some hexoses with xcex2[1xe2x86x924] linkages. Lignin is a complex random polyphenolic polymer. Therefore, lignocellulose represents a very cheap and readily available substrate for the preparation of sugars, which may be used alone or microbially fermented to produce alcohols and other industrial chemicals.
Ethanol, one of the alcohols, which can be produced from lignocellulosic biomass, has a number of industrial and fuel uses. Of particular interest is the use of ethanol as an additive to gasoline to boost octane, reduce pollution and to partially replace gasoline in the mixture. This composition is the well known commercial product called xe2x80x9cgasoholxe2x80x9d. It has been proposed to eliminate gasoline completely from the fuel and to bum ethanol alone. Such a fuel would produce considerably less air pollution by not forming as much carbon monoxide or hydrocarbon emissions. Furthermore, gasoline is produced from crude oil, which fluctuates in price, availability, and is the subject of unpredictable world politics.
It has been estimated that about 1xc3x97109 tons of lignocellulosic wastes are produced every year. This amount exceeds the total amount of crude oil consumed per year. In theory, if properly managed, the lignocellulose produced by the United States is sufficient to produce all of the country""s needs for liquid fuel if the cellulose and hemicellulose can be completely converted into ethanol. The amount of energy theoretically obtainable from the combustion of cellulose or the glucose or alcohol derived therefrom is about 7200 BTU per pound or roughly equivalent to 0.35 pounds of gasoline. Hemicellulose has similar value when converted into sugars or ethanol. Consequently, cellulose and hemicellulose represent a readily available potential source for ethanol production. The technology for the production of ethanol from grain and fruit for beverage purposes has been well developed for centuries. However, the costs have been relatively high compared to the cost of gasoline. Accordingly, many methods have been proposed to reduce the cost and increase the efficiency of ethanol production.
Among the techniques proposed for the production of fuel grade ethanol include the hydrolysis of cellulose and hemicellulose to produce sugars which can be fermented to produce ethanol. Cellulose in the form of wood, newsprint and other paper, forest, agricultural, industrial and municipal wastes is quite inexpensive compared to grain, fruit, potatoes or sugarcane which is traditionally used to prepare alcohol beverages.
Hydrolysis of lignocellulosic biomass using an acid catalyst to produce sugars has been known for decades but can be costly and requires special equipment. The hydrolyzed sugars themselves are somewhat labile to the harsh hydrolysis conditions and may be degraded to unwanted or toxic byproducts. If exposed to acid for too long, the glucose derived from cellulose degrades into hydroxymethylfurfural, which can be further degraded into levulinic acid and formic acid. Xylose, a hemicellulose sugar, can be degraded into furfural and further to tars and other degradation products.
In order for acid to completely hydrolyze the cellulose and hemicellulose in a lignocellulosic substrate, degradation of the desirable sugars and formation of the toxic byproducts cannot be avoided due to kinetic constraints. On the other hand, to use conditions sufficiently gentle that significant degradation of sugars will not occur does not result in complete hydrolysis of substrate. Furthermore, the acid is corrosive and requires special handling and equipment. Accordingly, in the last twenty years attention has focused on enzymatic hydrolysis of cellulose with cellulase followed by fermentation of the resulting sugars to produce ethanol which in turn is distilled to purify it sufficiently for fuel uses.
Cellulase is an enzyme complex that includes three different types of enzymes involved in the saccharification of cellulose. The cellulase enzyme complex produced by Trichoderma reesei QM 9414 contains the enzymes named endoglucanase (E.C. 3.2.1.4), cellobiohydrolase (E.C.3.2.1.91) and xcex2-glucosidase (E.C.3.2.1.21). Gum et al, Biochem.Biophys.Acta, 446:370-86 (1976). The combined synergistic actions of these three enzymes in the cellulase preparation completely hydrolyses cellulose to D-glucose.
However, cellulase cannot completely degrade the cellulose found in native, unpretreated lignocellulose. It appears that the hemicellulose and lignin interfere with the access of the enzyme complex to the cellulose, probably due to their coating of the cellulose fibers. Furthermore, lignin itself can bind cellulase thereby rendering it inactive or less effective for digesting cellulose. For example, raw ground hardwood is only about 10 to 20% digestible into sugars using a cellulase preparation.
2. Description of the Prior Art
U.S. Pat. No. 4,529,699 disclose a process for obtaining ethanol by continuous acid hydrolysis of cellulosic materials by providing a homogenized slurry of heated (160xc2x0 to 250xc2x0 C.) cellulosic material continuously into a reactor, adding concentrated acid to the pressurized and heated cellulosic material to obtain hydrolysis, neutralizing and fermenting the resulting aqueous solution to obtain ethanol, and recovering resulting byproducts of methanol, furfural, acetic acid and lignin.
A process for the production of sugars and optionally cellulose and lignin from lignocellulosic raw materials is disclosed in U.S. Pat. No. 4,520,105. The process entails subjecting vegetable materials to a chemical pretreatment with a mixture of water and lower aliphatic alcohols and/or ketones at 100xc2x0 to 190xc2x0 C. for a period of from 4 hours to 2 minutes with control of the breakdown of the hemicellulose components followed by separation of residue and a subsequent chemical treatment with a similar solvent mixture at elevated temperatures for a period of from 6 hours to 2 minutes.
U.S. Pat. No. 5,411,594 disclose a hydrolysis process system for continuous hydrolysis saccharification of lignocellulosics in a two-stage plug-flow-reactor system. The process utilizes dilute-acid hydrolysis and is primarily by reverse inter-stage transfer-flow, opposite to biomass, of second-stage surplus of: process heat; dilute-acid; and ingredient and solution water, all in an alpha cellulose hydrolysate, dilute-acid solution. The primary final product is the combined hydrolysate sugars in a single solution, including pentose, hexose and glucose sugars, which are fermented into ethanol and/or Torula yeast. The secondary final solid product is unhydrolyzed lignin solids.
A method of treating biomass material using a two-stage hydrolysis of lignocellulosic material is disclosed in U.S. Pat. No. 5,536,325. The conditions during the first stage is such as to hydrolyze or depolymerize the hemicellulosic component without substantial degradation of resulting monosaccharides and conditions during the second stage being such as to hydrolyze the cellulose to glucose without substantial degradation of the glucose. Hydrolysis in both stages is accomplished by the use of nitric acid, and the pH, retention time, and temperature in both stages are selected to maximize production of the desired monosaccharide or monosaccharides.
U.S. Pat. No. 6,022,419 disclose a multi-function process for hydrolysis and fractionation of lignocellulosic biomass to separate hemicellulosic sugars from other biomass components such as extractives and proteins; a portion of the solubilized lignin; cellulose; glucose derived from cellulose; and insoluble lignin from the biomass by introducing a dilute acid into a continual shrinking bed reactor containing a lignocellulosic material at 94xc2x0 to 160xc2x0 C. for 10 to 120 minutes at a volumetric flow rate of 1 to 5 reactor volumes to solubilize extractives, lignin, and protein by keeping the solid-to-liquid ratio constant throughout the solubilization process.
A process for rapid acid hydrolysis of lignocellulosic material is disclosed in U.S. Pat. No. 5,879,463. The process is a continuous process for acid hydrolysis of lignocellulosic material through which delignification and saccharification are carried out in a single reaction cycle employing a solubilizing organic solvent of lignin and a strong and extremely diluted inorganic acid to obtain highly concentrated recoveries of sugar.
There is a need in the art of obtaining fermentable sugars from lignocellulosic materials under mild conditions to prevent degradation of desirable sugars and formation of toxic byproducts normally obtained from acids used to completely hydrolyze the cellulose and hemicellulose in a lignocellulosic substrate, and yet avoid the incomplete hydrolysis of substrate resulting from sufficiently gentle acid hydrolysis that results in insignificant degradation of the sugars produced.
One object of the present invention is to provide milder hydrolysis conditions for lignocellulosic materials to reduce the cost of steam and equipment, limit degradation of desirable sugars and the formation of toxic byproducts due to kinetic constraints and to provide more complete hydrolysis of the lignocellulosic substrate.
Another object of the present invention is to provide milder hydrolysis conditions for lignocellulosic materials to reduce the cost of steam and equipment and limit degradation of desirable sugars and the formation of toxic byproducts, while providing more complete hydrolysis of lignocellulosic substrate, by employing a combination of a dilute acid catalyst and a metal salt catalyst.
A further object of the present invention is to provide milder hydrolysis conditions for lignocellulosic materials to reduce the cost of steam and equipment and limit degradation of desirable sugars and the formation of toxic byproducts, while providing more complete hydrolysis of lignocellulosic substrate, by employing a combination of a dilute acid catalyst and a metal salt catalyst to achieve both hemicellulose and cellulose hydrolysis in a single stage.
A yet further objective of the invention is to achieve a higher glucose yield in a two-stage dilute acid hydrolysis process.
In general, the lignocellulosic materials of the invention are hydrolyzed by subjecting the milled lignocellulosic material in a reactor to a catalyst comprised of a dilute solution of a metal salt and a strong acid to lower the activation energy (i.e. temperature) of cellulose hydrolysis compared to that normally required for dilute acid hydrolysis. The lower temperature obtained occasions a reduction in the cost of steam and equipment and enables the hydrolysis of both hemicellulose and cellulose to proceed in a single stage with higher glucose yield from cellulose and less degradation of the hemicellulose sugar. The lower temperature also occasions a higher glucose yield in the second-stage of a two-stage dilute acid hydrolysis process.